Positioning devices such as blockable gas springs and hydropneumatic springs are widely used, for example for height adjustment of tables and chairs. For blocking and unblocking such positioning devices a valve is provided between two fluid-filled working chambers of the positioning device. The valve is frequently operated through a rod or pin extending beyond a piston rod or a housing of the positioning device. The requirements as to the actuation of this actuating rod are different depending on the various constructions of which the positioning device forms a part. Sometimes a remote actuation is desired, sometimes a direct actuation of the piston rod is possible, sometimes a long actuation path is acceptable and a low actuation force is required. In other situations a high actuation force is available and a short actuation path is required.